Granular convection

https://en.wikipedia.org/wiki/Granular_convection

by glcheetham • 7 years ago

46 22 7 years ago

en.wikipedia.org

ptah• 7 years ago

this also has application in mixed spice grinders. you can shake it right side up or upside down to get the desired effect

tgb2• 7 years ago

My first instinct would be that this is due to entropy rather than minimizing energy, but the article doesn't mention that hypothesis. Is it easy to rule this out?

ggus• 7 years ago

Isn't "minimizing energy" a definition for entropy?

My idea is that, when shaking, all pieces try to move. Big pieces can only go upwards, while small pieces can go sideways (and take the space that was previously taken by a big piece). This ends up with the effect of big pieces floating to the top

zentiggr• 7 years ago

I use this to sort out favorite flavors and types of snack mixes all the time. Irritates my wife a little when I've eaten all of one type and left the rest.

jjoonathan• 7 years ago

Isn't "minimizing energy" a definition for entropy?

No. If you have a system with a well-defined energy/entropy exchange rate (temperature) you can use it to put entropy in units of energy, add it to your energy term, and model the combined system behavior on the basis of this "free energy." The free energy is not a fundamental quantity, though, and it doesn't define entropy. That's why the first two laws of thermodynamics are "Energy is Conserved" and "Entropy Increases," not "Free Energy decreases."

amluto• 7 years ago

Isn't "minimizing energy" a definition for entropy?

No. I’m not really sure how you’re trying to define entropy, but you’ll probably have quite a bit of difficulty with endothermic processes if you try to think about entropy like this. For example, if you sprinkle sodium acetate in water, the sodium acetate dissolves, and the solution absorbs heat in the process, thus increasing the energy of the system.

In general, entropy increases but energy just moves around.

itronitron2• 7 years ago

the Brazil nut effect results in less randomness in the system as larger items rise to the top and smaller ones descend

pletnes• 7 years ago

The system isn’t closed - you’re exchanging energy and entropy with the shaker.

tgb2• 7 years ago

That makes sense, thanks for resolving it for me.

uoaei• 7 years ago

It's about maximizing density, which corresponds roughly to 1) minimizing energy by making the aggregate have as low of a center-of-gravity as possible, and 2) minimizing entropy of the arrangements of the components. Often energy and entropy minimization coincide quite closely but don't exactly share the same minimum nor optimization path.

phkahler• 7 years ago

Farmers know the rocks come to the suface over the winter.

itronitron2• 7 years ago

interesting, I suppose if the top 6 inches of a field freezes then it slightly expands in volume and when the soil thaws the smaller soil particles would then have an opportunity to fall beneath the rock... not sure if this is dependent on the top or bottom thawing first though...

no_protocol• 7 years ago

In many places, the frost line is more like 3 to 6 feet, than 6 inches.

dekhn3• 7 years ago

I remember a journal club where we read a paper in which they demonstrated the inverse brazil nut effect (http://large.stanford.edu/courses/2007/ph210/spector2/). The scientists built a physical nut shaker- like, a huge tube that they bounced. that's when I realized I had picked the wrong field (theoretical biology instead of experimental physics).

technoplato• 7 years ago

What do you do in theoretical biology?

dekhn3• 7 years ago

Nothing anymore- I became disillusioned with modern scientific research, especially biology, because I believe that most of what is published is simply false or wrong (it's much harder to publish false crap in experimental physics because everybody knows more math, is better at analyzing experiment design, and the systems are "simpler"). Now I work on software engineering related to neural network performance (focusing on things that have worked well for years).

That said, things I've worked on include: classifiers to identify genes in DNA, molecular dynamics simulations of proteins and nucleic acids, methods to discover new drugs, analysis of protein function from an evolutionary perspective, protein design.

cr0sh• 7 years ago

Here I was this morning just thinking about this - weird how that happens.

My wife got this gift recently, that at first looks like a snowglobe, but actually has fine sand and seashells inside. You can kind of shake it or rotate it about, and the seashells that "rise" to the top on the sand change as you do it.

For some reason, that got me thinking about this topic. Then I see it on HN...

Abishek_Muthian• 7 years ago

Maybe OP bought pack of assorted nuts for Christmas and wondered the same.

malcolmhere• 7 years ago

This is a recurring problem in the animal feed industry. Easiest explanation for the phenomena: the small particles fall through the gaps gaps between the large particles.

Granular convection

Related Stories